Risk of Rain Vanilla Acrid
Acrid is either a bit of an in-between class or a whole thing to himself depending on how you want to look at it; most classes in Risk of Rain are either primarily ranged fighters who want to stay out of the mess of mobs or primarily melee fighters who want to hurl themselves amid groups and kill everything at once. Acrid's basic attack is melee, but he's really more an in-and-out sort of class, laying down slime trails to damage enemies pursuing him and harrying them with his ranged options, with his basic melee attack being more filler than a defining capabilities. You can play him more-or-less like the other melee classes if you get the right item support, unlike most of the ranged classes, but it's not really his thing. Yet true ranged combat isn't really his thing either, with only one genuinely long-ranged attack in his arsenal that has a long cooldown and is only really effective against clustered mobs.
Acrid is also surprisingly durable (High base HP, innate armor rating where most characters don't start with armor), with particularly swift HP regeneration, contributing well to his preferred hit and run strategies.
Unlock-wise, you've already seen how he gets unlocked; open the crate and kill the Acrid boss that pops out. There you go.
Acrid's four moves are...
Festering Wounds: Acrid steps forward slightly and bites or slashes one target for 120% base damage, and also temporarily poisons the target for 24% base damage per second. The poison self-stacks. No cooldown.
Neurotoxin: Fires a mid-ranged blast in a straight line that hits every enemy it touches for 210% base damage and stuns them. Penetrates terrain. 3 second cooldown.
Caustic Sludge: For two seconds, Acrid generates a trail of caustic sludge so long as he is touching the ground. This sludge speeds up allies that walk on it, including Acrid himself as he generates the sludge, while enemies touching it are slowed and take 90% base damage per second. Once laid, a given section of sludge lasts 15 seconds. 17 second cooldown.
Epidemic: Lobs a projectile in a straight line, penetrating terrain and going a fair distance beyond the edge of the screen. Once the projectile hits an enemy, that enemy becomes infected, taking 100% base damage per second. After a second has passed, Epidemic will attempt to spread from a victim to two other victims within a small radius in any direction straight through terrain, who can then spread it to two other victims, and so on, though a given use of Epidemic cannot infect a given target more than once. Additionally, if an infected target dies, its infection will linger for a moment and potentially jump to another target. 11 second cooldown.
Acrid really comes into his own as the game intensifies and it becomes increasingly routine for you to be mobbed. Early in the game you'll probably just tear apart lone enemies with Festering Wounds, but Acrid is really best off having groups of enemies chase him into Caustic Sludge for mass damage, hurling Epidemic to wear down groups, and getting in Neurotoxin hits on clusters for it to be worth getting that close. Festering Wounds itself is, past the early game, primarily a way of wearing down big tough enemies that aren't too hard to avoid the attacks of or whose damage output isn't that high. Mostly this amounts to bosses, but can include some elites depending on what the base enemy type is and what the elite bonus is.
As such, an important part of playing Acrid well is having good judgment about when to ignore a few enemies as not worth your time vs when to actually engage groups. As it happens, I don't really have good judgment in this regard -I like playing Acrid and think he's great, in fact he's actually the first class I almost beat the game with, but as this video illustrates I tend to push too hard for fights even when they're not actually worth the payoff. In this run luck was partly to blame, with the first level being heavy on things to spend money on, encouraging me to grind for money for a bit, but this really is mostly on me. I'm much better with some of the other classes.
A related point is that Acrid is both actually fairly hard to get too deep into trouble -the bigger the horde of enemies chasing you, the more damage you're dishing out- and yet fairly easy to overestimate your capabilities and end up behind the curve. Killing enemies per se isn't worth much past the early game; collecting items is power, and aside boss drops and a few other caveats once you've opened all the chests, prayed out all the shrines, and so on, you really should be moving on, and in fact should have triggered the teleporter before you finished with all that. I misjudged that on the snow level, and ended up with ludicrous gobs of cash but still behind the curve.
This run also shows off a bit of how Acrid tends to not do so well with Drones. His habit of ducking in and out of conflict often gets his drones beaten up even as Acrid himself is jumping out of the way of enemy attacks, since if a Drone is neither Medical nor Missile it tries to hover down near the enemy and thus in their strike range, unless the player has a fair amount of distance between them and the other enemies. So... they tend to die. Which is too bad, since they're fairly synergistic with his style, making it so you're stacking on even more damage even as you're fleeing your enemies.
On a completely unrelated note, while I was working on recording a successful Acrid run, I came to realize the Hyper-Threader was causing extreme lag that didn't fully clear up until the game was outright closed. I have a Bandit recording that in-game took an hour and ten minutes, but out of game took an hour and 45 minutes, dropping to 10 frames per second toward the end, and double-checking it confirmed that it was fine until it got the Hyper-Threader and, though grabbing it wasn't an immediate problem, it did lag once I started triggering its shots. The Bandit recording I actually put up was my second recording of a successful Bandit run, because oh god no. So I'll be avoiding the Hyper-Threader in future, both for audience sake and for my own sanity.